A natural hazard is an extreme natural phenomenon that threatens human lives, activities or property, or the environment of life. Natural disasters are the destructive consequence of extreme natural hazards, and globally there are more than 700 of them each year. Floods are the most common natural disaster. Together with earthquakes and cyclonic storms they are the most destructive of such manifestations.
Natural phenomena may be transformed into hazards either by excess or by dearth. For example, too great a discharge of water may give rise to flooding, whereas too little may cause a drought. A situation becomes hazardous when the physical forces or environmental stresses at work exceed the ability of human social, economic, cultural, or health systems to absorb, resist, or avoid the resulting negative impact. In this respect, natural hazards are defined not only by the natural forces that induce them, but also by the vulnerability of human systems. Vulnerability is defined here as the susceptibility of people or things to harm.
The threat of a natural hazard is either constantly present or is subject to fluctuations. Many hazards are cyclical; for example, earthquakes of a certain size will occur on a given fault when enough tectonic stress has been accumulated to overcome the frictional resistance of the rock mass to slipping, a process which will probably occur with a definable time interval because of the gradual build-up of strain on the fault. Other hazards, especially meteorological ones, may be seasonal.
Generally, the vast majority of hazards are subject to a rule of magnitude and frequency in which the higher the magnitude, the lower the frequency of occurrence. Some hazards, such as volcanic eruptions, may operate on a geological timescale that is much longer than the scale of human lives. In such cases it can be very difficult to justify the allocation of resources to prepare effectively for events that have a low probability of occurrence during a single human lifetime.
In other cases, the repetitiveness of a hazard may be a problem. For instance, the solvency of the U.S. National Flood Insurance Program (NFIP) depends as much on reducing the instance of repeated claims as it does on anticipating and reducing the impact of large, infrequent events. In a small number of cases, claims have been made for reimbursing damage to a single property up to five times in a decade. Such problems must be abated by reducing either the hazard or the vulnerability to it.
In everyday situations the product of hazard and vulnerability is risk, which can be defined as the probability or likelihood that an event of a given kind and size will occur in a given interval of time and with an anticipated set of negative consequences. Engineers tend to define risk by calculating numerical values of the probability, while social scientists may be more interested in how risk is perceived and how some of the intangible features of human behavior affect it. In any event, paradoxically, risk is a hypothetical quantity (though no less important for that). It materializes as impact, which should lead to an emergency response that reduces the harm done as much as possible. Hence:
Exposure to natural hazards becomes an issue when an item (such as a person, a community, a building, or an economic activity) is not constantly at risk. Despite temporal variations in strain upon Earth’s crust, to all intents and purposes we may consider earthquake risk to be fairly constant, especially as it cannot accurately be predicted in the short term. However, predictable hazards such as hurricanes, which can be monitored and tracked before they make landfall (i.e., arrive at a coast), may allow a forecast to be turned into a warning that stimulates an organized response on the part of the threatened community. Generally, where it is feasible, evacuation is the most effective means of reducing the exposure of people to death or injury in high magnitude events.
The question of exactly what phenomena should be classified as natural hazards has long been debated by students of the field. The core phenomena consist of geophysical events from the atmosphere, hydrosphere, and geosphere (the lithosphere), and to a lesser extent from the biosphere. Earthquakes, landslides, and subsidence of the ground are geospheric hazards of the first order; tropical cyclones (also known as hurricanes and typhoons), tornadoes, and windstorms are the leading examples of meteorological hazards; and drought and floods are the principal threats from the hydrosphere, with subdivision of the latter into riverine, rain-fed, coastal, and glacial outburst forms.
By convention, though not necessarily on the basis of any very robust theoretical reasoning, disease outbreaks in humans, animals, and plants (i.e., epidemics, epizootics, and epiphytotics) are not usually classified as natural hazards. However, locust infestations are often included.
A further definitional problem occurs when disasters have mixed natural and human-induced (anthropogenic) causes. For example, destructive floods can result from dam bursts, which can in turn result from excessive river flows, earthquakes, or rapid landslides or snow avalanches that cause water waves in the reservoir, if not from failure of the materials or design of the dam itself. In point of fact, natural hazard and natural disaster are convenience terms. Whatever one’s religious convictions, responsibility for damage and destruction cannot be shrugged off by referring to unpredictable "acts of God," as they stem from failure to mitigate forms of human and environmental vulnerability that are well known and understood.
In conceptual terms, serious study of natural hazards began in the 1920s with the development of the "human ecology" field. From 1945 onward the Chicago school founded by Harlan Barrows (1877-1960) and taken forward by Gilbert Fowler White (1911-2006) gradually revealed the human perceptual and social processes of adjusting to hazards. White and his students found a rich source of study in the struggles of U.S. Great Plains farmers to adapt to varying patterns of drought and flood. By and large, research in many other parts of the world has confirmed the findings of the U.S. human ecologists and geographers, despite some variations due to cultural differences. Thus, the "hazardousness of place" is tempered by the choice of adjustments that people who inhabit zones of hazard are able to employ.
The Chicago school was motivated to explain why structural responses had not solved the problem of natural hazards. For example, a century of canalization and levee building by the U.S. Army Corps of Engineers on the Mississippi River ended in 1993 with the worst and most prolonged flood on record. Clearly residents, developers, and planners on the floodplain had made some false assumptions about the infallibility of structural flood defenses.
With some success White and his colleagues advocated an approach based on a mixture of structural and nonstructural protection. It may still be necessary to build barriers to stop flooding, or to strengthen buildings so that they resist earthquakes, but it is equally necessary to tackle such hazards with organizational methods. Hence the nonstructural solutions include evacuation (where feasible), emergency planning, land-use control, and public awareness campaigns.
Unfortunately, despite the best efforts of mitigation specialists, the world has not become less susceptible to hazards over the last half-century. For example, Hurricane Katrina, which made landfall in Louisiana and Mississippi on August 29, 2005, killed 1,848 people, seriously damaged or destroyed 78,000 homes, and left more than half of the population of New Orleans without shelter. As Hurricane Ivan had narrowly missed crossing the city a year previously, the scenario for a major storm impact was well known. Despite this, the heights and state of maintenance of levees were insufficient, as were evacuation and recovery plans. Failures of coordination between local, state, and federal levels of government led to a relief debacle. Rebuilding will probably take eight to eleven years and, due to the phenomenon of geographical inertia (the reluctance of long-term residents to relocate their homes and businesses), will necessarily require considerable investment in major yet fallible structural defenses.
The relentless rise in global population, polarization of wealth between rich and poor, marginalization of vulnerable communities, and the prevalence of about twenty-five complex humanitarian emergencies have all contributed to the increasing toll of natural disasters. So has the increasing complexity and interdependence of modern society, and so, no doubt, will global warming and climate change, as more extreme, if not more frequent, meteorological phenomena are likely to occur.
The average annual death toll in natural disasters is about 140,000, but there is very considerable variation from one year to another. In fact, after five years in which the death toll averaged about 58,000, the Asian tsunami of December 26, 2004, took at least 230,000 lives. Despite the irregularities, there are discernible upward trends in the number of people directly affected by natural disasters (at least 250 million a year) and the cost of disasters (well in excess of US$100 billion a year), although improved protection has had some effect in stemming the rise in mortality.
Despite much debate and many good intentions, global vulnerability to natural hazards remains unacceptably high. Critical facilities—schools, hospitals, essential lifelines—remain heavily at risk in many countries (for example, in the Kashmir earthquake of October 5, 2005, schools frequented by 48,000 children collapsed). More money continues to be spent on responding to disasters than on reducing the risks of future ones. Although vulnerability and poverty are not precisely synonymous, in both rich and poor countries they are very closely linked. Hence natural disaster impacts involve serious questions of equity. Natural hazard impacts need to be mitigated by a mixture of prevention, avoidance, and sustainable development: In short, sustainable disaster reduction is required.
